The present invention is in the field of high resolution scanning techniques and relates to an apparatus for near-field optical inspection of articles, particularly useful for inspecting a substantially large surface area with very high spatial resolution.
There is a great variety of scanning systems having a common goal of increasing the system""s resolution. It is known that the resolution depends on the diameter of a light beam striking the object, namely the less the diameter of the beam, the higher the resolution of the system. Near-field applications such as optical data storage, inspection, microscopy, etc., solve the resolution problem by generating a point-like light source, having dimensions smaller than the light wavelength. This is typically achieved either by defining small apertures on opaque screens, or, alternatively, by passing the light through point-like tips of sub-wavelength dimensions. The tips (constituting point-like light sources) are located at very close proximity of the object, in order to provide high optical resolution of the scanning system.
One example of a near-field optical microscope that utilizes a point-like tip and allows for inspecting a sample with substantially nano-scale resolution is disclosed in U.S. Pat. No. 4,947,034. A light beam illuminates the tip and a portion of light striking the tip scatters and forms local evanescent fields from the very end region of the tip to the sample surface, which is in proximity to the tip. The evanescent fields very close to the tip interact with the surface atoms of the sample. The optical properties of the sample""s surface are measured in the following manner. First and second dither motions of different frequencies are applied to the tip relative to the sample (or vice versa) in directions, respectively, normal and parallel to the plane defined by the surface of the sample. Then, light scattered from the end of the tip and the sample is detected. In order to measure the entire surface, the sample is supported for movement in the X and Y directions beneath the stationary tip. The motion of the sample relative to the tip is controlled by X- and Y-piezoelectric drives, while the oscillation of the tip in Z-direction is controlled by Z-piezoelectric drive.
Another example of an apparatus for near-field optical microscopy is disclosed in U.S. Pat. No. 5,018,865. A photon scanning tunneling microscope is developed using a phenomenon of sample-modulated tunneling of photons in a near field to produce information about the sample. The evanescent near-field is produced by utilizing the effect of total internal reflection of a light beam incident on an interface between the materials of different refraction indices, when the incident beam lies in the medium of higher index. The intensity of this near field increases perpendicular to and towards the surface of the sample and has substantially constant intensity in a plane substantially parallel to the surface. To this end, the sample is placed within the near field and the presence of that sample changes the intensity distribution within the near-field, which is probed by an optical fiber probe tip. Photons from the incident beam tunnel through the region between the tip and the sample and can be collected by a suitable detection system. The intensity of the near field is measured adjacent to the sample, the measurement producing an image corresponding to the measured area of the sample. A feedback circuit is employed to regulate the intensity of the signal by varying the tip to sample distance, preventing thereby the tip from contacting the sample.
U.S. Pat. No. 5,508,805 discloses an optical scanning type tunneling microscope utilizing an optical probe. According to this technique, reference light is provided in addition to light projected to a sample, and an effect of interference between the reference light and light picked up from an optical probe is utilized to obtain phase information of light about a region on the sample having very small dimensions (smaller than a wavelength of emitted light). The optical probe is formed with a tip-like projection on its surface facing the sample. The tip serves for picking up or extracting evanescent waves, which are generated on the surface of a glass substrate supporting the sample and around the sample, wherein the generation of these evanescent waves is caused by the incidence of measuring light upon the glass substrate. This optical probe comprises three fused single-mode optical fibers, the first fiber serving for transferring the reference light into the second fiber optically coupled with the first fiber, and the third fiber serving for detecting a relative position of this fiber relative to the surface of the sample. The tip-like projection is formed on the distal end of the second fiber, which thereby conveys both the transferred reference light and the collected, measured light.
European Publication No. 0507628 discloses a near field scanning optical microscope aimed at improving the resolution of the microscope by utilizing a light source emitting light in a pulse-like manner, an optical probe (near field optical means), and a detector operable in synchronism with the pulsed light from the light source. The optical microscope may also utilize an STM (or AFM) that detects data on the surface relief of the sample. The output of the STM can be used for maintaining a constant distance between the optical probe and the surface of the sample. To this end, however, the STM and the optical probe are kept at a known distance from each other, and the output of the STM indicative of the surface relief is recorded to be used for positioning the optical probe when reaching a specific location on the sample (the recorded recess or projection).
Most of the inspection systems of the kind specified, especially those used for inspecting patterned articles, are aimed at covering a large surface area (hundred square centimeters in a few minutes). The term xe2x80x9cpatterned articlexe2x80x9d signifies an article formed with regions (features) having different optical properties in respect of incident radiation. The use of a plurality of nano-probes (tips) seems to be the simplest solution of achieving both goals, namely increasing the resolution and measured area. Unfortunately, this approach does not ensure reproducible results because it scales the cost linearly with the number of probes, wherein each such nano-probe has its own characteristics. This requires a very complicated image processing technique, if any, for successfully mapping the surface area scanned by the entire probes.
U.S. Pat. No. 5,633,972 discloses an aperture-based imaging fiber for generating a plurality of subwavelength light energy beams concurrently to be used for near field viewing. The use of such a fiber for illuminating a specimen is aimed at reducing image acquisition time.
There is accordingly need in the art to overcome the disadvantages of the conventional techniques by providing a novel apparatus for near-field optical inspection.
It is a major object of the present invention to provide such an apparatus that enables to obtain simultaneously high-resolution image from a large area.
It is a further object of the present invention to provide such an apparatus that allows for combining the principles of optical and scanning tunneling microscopy for successfully inspecting articles.
There is thus provided according to the present invention an apparatus for optical inspection of an article utilizing near-field illumination, the apparatus comprising:
(a) a light source unit generating incident light for illuminating the surface of the article;
(b) a detector unit for sensing light signals and providing data representative thereof;
(c) a fiber bundle for directing said incident light onto a substantially large surface area of the article and collecting light returned from the illuminated surface area; and
(d) a control means for adjusting the position of the fiber bundle relative to the surface of the article.
The main idea of the present invention consists of the following. The fiber bundle is used for directing the incident radiation onto the surface of the article to be inspected. The fiber bundle comprises a group of illuminating fibers extending between the light source unit and the surface, generating thereby the plurality of point-like light sources illuminating together the substantially large surface area of the article. These point-like light sources are located in a plane defined by an outer surface of the fiber bundle close to the surface of the article. Additionally, the fiber bundle comprises a plurality of light connecting fibers extending between the surface of the article and the detector unit for directing light returned (scattered or emitted) from the illuminated area to the detector unit. It is understood that the desired vertical position of the plane, in which the point-like light sources are located relative to the article, should be maintained so as to ensure the proximity of the bundle to the surface of the article under inspection. To this end, the control means is constructed and operated so as to utilize the principles of either scanning tunneling microscopy (STM) or atomic force microscopy (AFM). The control means comprises at least three small sensing tips attached to the fiber bundle for operating in either STM or AFM mode, and a feed back electronic loop.
Preferably, the ends of the illuminating and collecting fibers located in the close proximity of the article are mixed so as to still more increase the illuminated area.
The advantages of the present invention are thus self-evident. On the one hand, the provision of the fiber bundle enables both the resolution and scan area to be significantly increased. On the other hand, the provision of at least three tips using STM or AFM mode allows for successfully detecting and desirably adjusting the vertical position of the entire plane of location of the plurality of point-like light sources relative to the article.